Overview:
The standard hot-wire anemometer functions based on a Wheatstone Bridge with one resistor replaced by the probe wire. By sampling the voltage across the bridge the fluid velocity at the sensor can be determined. The Pule Width Modulated - Constant Temperature Anemometer applies a set voltage until the wire reaches the desired overheat, the duty cycle of this pulse is then sent back to the PC as an integer number. Overall, the PWM-CTA presents advantages in speed, cost and data analysis over the traditional designs.
Benefits:
- Digital output eliminates the need for additional analog to digital conversion equipment
- Up to 18 independent channels synchronously sampled
- Increased sensitivity compared to a conventional CTA. This is due to the second order response of the PWM-CTA vs. fourth order response of a standard CTA.
- Frequency response can be made proportional to velocity
- PWM-CTA control system is inherently stable
- Relative cost per channel for the PWM-CTA is significantly lower than an analog CTA with similar capabilities.
Features:
- Windows graphical user interface
- Synchronous sampling on up to 18 channels.
- All channels independently configurable
- Trigger in and trigger out for synchronization with other measurement devices
- Sample frequency of 100kHz, 50kHz 33.3kHz
- User Configurable channel operation - all channels can be configured and
run independently.
- Pulse Width Modulation for driving a hotwire. Allows gain-bandwidth, slew rate, and gain adjustments for noise elimination
- Resistance measurement 0 - 10 Ohms for cold resistance of hotwires
- Analog voltage measurement for synchronously sampling from other devices. User adjustable input voltage range, ±10V, ±5V, ±2.5V, ±1.25V
Disclosure:
The currently available electronic implementation of the PWM-CTA exhibits a modest attenuation in its output for frequencies ≥ 3kHz. A representative example ofthis attenuation is shown in the figure below. These data were taken in the sheer layer of a jet: ū = 5.25 m/sec. We are currently working to increase this range.
PWM-CTA high frequency attenuation
Pictures:
Conventional constant temperature anemometer schematic
PWM-CTA functional schematic
Eighteen channel PWM-CTA. Each vertical set of connectors is one channel.
Screenshot of the PWM-CTA computer control interface.
